Modern communication networks often show a “decomposed” or layered architecture, in which the call and session control layer and the media plane layer are handled by different instances that are typically realized as separated physical nodes. Typically the nodes of the call and session control layer are referred to as media gateway controllers and the nodes of the media plane layer are referred to as media gateways.
A protocol widely used in the control layer for controlling sessions consisting of one or several media streams is the Session Initiation Protocol SIP being specified in the Internet Engineering Task Force document “IETF RFC 3262, Session Initiation Protocol” in the following being referred to as RFC 3262.
A further protocol defined by the IETF, the Session Description Protocol SDP specified in the IETF document RFC 4566, is used in a variety of networks and communications systems for describing multimedia sessions, e.g. for the purpose of session announcement, session invitation and other forms of multimedia session initiation. Examples of such networks are the IP Multimedia Subsystem—IMS— and the Multimedia Telephony Service—MMTeI—. The SDP is thereby used in combination with the SIP in a way that SDP messages or packages are embedded into SIP.
Since its origins, SDP has evolved with new capabilities to respond to the needs of new applications, as corresponds to a constantly growing use of the protocol. A capability to group different media has been specified for different applications in various IETF documents. By means of media grouping in SDP, a particular relationship between two or more media streams can be indicated.
The framework for grouping of media is specified in the IETF document RFC 5888 that also specifies specific applications for the use of this capability, and further in the IETF document RFC 4588 titled “RTP Retransmission Payload Format”, and the IETF document “Simultaneous Media Grouping, draft-johansson-mmusic-grouping-simultaneous-00” of Apr. 28, 2010, in the following being referred to as DRAFT SID.
Examples for the use of media grouping are:                Synchronized play-out of the different media streams, using so-called “LS” semantics, (specified in RFC 5888),        Simultaneous media grouping of media streams that constitute different representations of identical content, for example a high resolution and a low resolution of the same video sequence sent to a video conference server using so-called “SID” semantics, (specified in DRAFT SID), and        RTP retransmission, wherein original media and replica of the media are grouped, for increased robustness using so-called “FID” semantics, (as described in RFC 4588)        
In essence, the mechanism makes use of the following two attributes:                The mid-attribute is used to identify each of the media blocks within a multiple media SDP session included at media level, i.e. within the m-block:“a=mid:” identification-tag        The group-attribute is used to indicate which of the media in the multiple media SDP session are grouped and for what purpose, being included at session level, i.e. “above” all m-blocks:“a=group:” semantics (SP identification-tag)        
Further typically, a protocol used between the control nodes and the media plane nodes is specified in ITU-T Recommendation H.248.1, titled Gateway Control Protocol (current Version 3), in the following also being referred to as media gateway control protocol or simply as H.248.
Whereas in SIP, embedded SDP is supported in its entirety (as specified in RCF 4566), the use of SDP in H.248 encounters some difficulties. In other words, it is not always possible to forward a SDP element from SIP to H.248 or vice versa.
Specifically, whereas SIP allows embedding an SDP element comprising a description of a plurality of media and a grouping of the media or of selected media out of the plurality of media, such SDP element cannot be embedded into H.248. The reason is that the different media blocks are separated into different stream descriptors (each referring to one m-block or, in other words, one m-line). Within H.248 each stream of the multiple streams has to be described by a separate stream descriptor. Thus SDP would have to be contained within a so called stream descriptor representing a single, bidirectional media stream.
It is to be noted that, formally, mid attributes could be included in the media blocks. However, lacking the instructions of what kind of grouping is required (in absence of the a=group line), a media gateway (e.g. MRFP) would disregard this information. In other words, current H.248 lacks a means to providing a group attribute having an influence beyond the m-block border.